Serial data streams are regularly subjected to a spread spectrum clocking (SSC) modulation scheme in order to lower the magnitude of the electromagnetic interference (EMI) radiation, which would otherwise be emitted at a single frequency in electronic products. For example, hard disk drives, personal computers, and computer monitors, among other electronics, use SSC modulation to lower EMI.
A common SSC modulation scheme implements a simple triangular frequency modulation form, which down-spreads the center frequency of a data signal's fundamental from −5000 to 0 parts per million (PPM). FIG. 1A shows an example of the resulting ideal shape of the frequency versus time trace associated with this approach. This type of trace would correspond to, for example, a laboratory grade 5000 PPM down-spread SSC profile used with a second generation serial advanced technology attachment (SATA) hard disk drive signal.
Consumer-grade clock generators often perform poorly in terms of stability and tend to generate problematic signals that can deviate outside of what is permitted based on specifications or other definitions. This can result in spikes or other deviations in the signal, such as spike 10 illustrated in FIG. 1B. Other problematic deviations include, for example, the “Batman” profile 12 shown in FIG. 1C, the “1:100 SSC glitch” profile 14 shown in FIG. 1D, and the “Noisy SSC” profile with transition fringes 16 as shown in FIG. 1E.
The electronics industry is becoming more aware of the problems caused by errant SSC modulated signals. Efforts to better define what constitutes a spike or a deviation in an SSC signal have been proposed. For example, a “dF/dT” (delta Frequency/deltaTime) definition has been put forward, which can be used on the demodulated SSC signal to define errant spikes. For instance, in FIG. 1F, a desired modulated SSC signal is shown as line 108. However, a spike having a first component 102 and a second component 104 occurs at position 120. Position 120 can correspond to X % of a generally sloped section 108 of the demodulated SSC signal, as shown in FIG. 1F.
The first component 102 of the spike or deviation corresponds to +df/dt and the second component 104 corresponds to −df/dt, or in other words, the positive and negative sloping components of the spike. An errant or deviant spike can be defined by whether the magnitude of the spike exceeds a value, such as 1875 PPM, within a moving average time interval (e.g., 106). The time interval is sometimes defined as 1.5 microseconds.
Various specifications and protocols such as SATA, serial attached SCSI (SAS), DisplayPort, and PCI Express, have incorporated limitations for SSC signal deviations, and signals falling outside of this or similar definitions are considered “non-conformant.” However, conventional real-time (RT) oscilloscope trigger systems do not have the ability to detect such errant events as they happen. And since dF/dT spikes can occur irregularly, it would be desirable to immediately isolate such events without searching a long record capture in the hopes of finding the event. Searching captured records is computationally expensive and can take on the order of minutes. Moreover, conventional RT oscilloscopes have an extremely low probability of having infrequent events in acquisition memory, which results in overly-conservative jitter reporting.
Therefore, it is difficult or impossible to diagnose dF/dT problems in the SSC signals, or other types of signals, or to make complete and timely determinations of the presence or absence of deviations in the signals.